Construction of ZnIn2S4-In2O3 Hierarchical Tubular Heterostructures for Efficient CO2 Photoreduction

Article abstract of DOI:10.1021/jacs.8b02200

We demonstrate the rational design and construction of sandwich-like ZnIn₂S₄-In₂O₃ hierarchical tubular heterostructures by growing ZnIn₂S₄ nanosheets on both inner and outer surfaces of In

Full text of DOI:10.1021/jacs.8b02200

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Construction of ZnIn2S4-In2O3 Hierarchical Tubular
Heterostructures for Efficient CO2 Photoreduction
Sibo Wang, Bu Yuan Guan, and Xiong Wen (David) Lou
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b02200 • Publication Date (Web): 29 Mar 2018
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Construction of ZnIn S -In O Hierarchical Tubular Hetero-
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3
structures for Efficient CO Photoreduction
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Sibo Wang, Bu Yuan Guan, and Xiong Wen (David) Lou*
School of Chemical and Biomedical Engineering, Nanyang Technological University, 62 Nanyang Drive, Singapore 637459,
Singapore
Supporting Information Placeholder
1
7-24
research areas.
Recently, hollow architectures have been
ABSTRACT: We demonstrate the rational design and
5
-8
studied as photocatalysts for CO reduction. The hollow
construction of sandwich-like ZnIn S -In O hierarchical
2
2
4
2
3
cavity not only possesses reduced bulk-to-surface diffusion
length to accelerate electron-hole separation, but also pro-
tubular heterostructures by growing ZnIn S nanosheets
2
4
on both inner and outer surfaces of In O microtubes as
2
3
vides large surface area and abundant active sites to boost
photocatalysts for efficient CO2 photoreduction. The
unique design integrates In O and ZnIn S into hierar-
21
CO adsorption and surface catalysis. Meanwhile, optical
2
2
3
2 4
absorption could be increased by multi-light scatter-
chical one-dimensional (1D) open architectures with dou-
ble-heterojunction shells and ultrathin two-dimensional
23
ing/reflection in the interior void. Among the various con-
figurations of hollow scaffolds, tubular structures with two
open ends may offer opportunities for hybridization with
other semiconductors on both surfaces of the shell to form
three-layered heterostructures. In addition, growing 2D sem-
iconductor nanosheets on tubular substrates is favorable to
reduce diffusion distance for charges and enhance exposed
(
2D) nanosheet subunits. This design accelerates the sep-
aration and transfer of photogenerated charges, offers
large surface area for CO adsorption, and exposes abun-
2
dant active sites for surface catalysis. Benefitting from the
structural and compositional merits, the optimized
ZnIn S -In O photocatalyst exhibits outstanding perfor-
4
,10
2
4
2
3
catalytic-active sites.
Therefore, it might be desirable to
mance for reductive CO deoxygenation with considerable
CO generation rate (3075 μmol h g ) and high stability.
combine these considerations mentioned above and prepare
2
-1 -1
novel photocatalysts to explore their benefits in CO
reduction.
photo-
2
Solar-driven CO reduction, a key process in artificial pho-
2
tosynthesis, has shown promise to produce carbon fuels and
1
relieve climate change. Developing efficient photocatalysts is
desirable to improve CO photoreduction performance for
2
practical implementation. Due to the distinct physicochemi-
cal features, metal oxide and sulfide semiconductors together
Figure 1. Schematic illustration of the synthetic process of sandwich-
like ZnIn -In hierarchical tubular heterostructures: (I) thermal
annealing in air, (II) growth of ZnIn nanosheets.
with their appropriate composites are promising photocata-
2-13
S
2 4
2 3
O
lysts for CO conversion. However, the achieved efficiency
2
2 4
S
so far is still not satisfactory mainly because of the sluggish
separation/transfer kinetics of electron-hole pairs, especially
when using bulk photocatalysts.
Herein, we demonstrate the rational design and fabrica-
tion of sandwich-like ZnIn -In hierarchical tubular het-
erostructures by assembling ZnIn nanosheets on both
inner and outer surfaces of In microtubes as double-
heterojunction photocatalysts for efficient visible-light CO
S
O
2 3
2
4
Delicate design of heterojunction is an effective strategy to
improve photocatalytic activity, owing to the rapid separa-
S
2 4
O
2 3
1
4-16
tion of charge carriers induced by built-in electric field.
Besides, coupling of proper semiconductors can also render
2
reduction. Figure 1 illustrates the overall synthetic processes
of the complex hollow heterostructures. First, hexagonal
1
1-13
enhanced photoabsorption and photostability.
Although
diverse heterostructures have been developed for photoca-
talysis, most heterojunction photocatalysts are bulk materials
with poorly controlled nanodomains of different species.
These conventional solid architectures still experience seri-
ous charge recombination in the bulk phase, thus limiting
the photocatalytic performance.
To maximize the virtues of heterojunction for photocataly-
sis, the fabrication of photocatalysts with proper nanostruc-
tures is of importance. Hollow particles with unique struc-
ture-dependent merits have shown advantages in diverse
In
O tubes are obtained through a thermal annealing treat-
2 3
25
ment of In-MIL-68 prism precursor (step I). Then, two lay-
ers of ZnIn nanosheets are grown on both surfaces of as-
derived In O tubes via a hydrothermal reaction (step II). The
2 3
14
S
2
4
photocatalytic functions of In O and ZnIn S are integrated
2 3 2 4
into hierarchical 1D hollow architectures with double-
heterojunction shells and ultrathin 2D nanosheet subunits,
which can assist the separation and migration of photogen-
erated charges, provide large surface area for CO
and expose plentiful reactive sites for surface reactions. Ac-
adsorption,
2
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cordingly, the optimal ZnIn S -In O photocatalyst displays
layers and In O shell (Figure S8e,f), suggesting the intimate
2 3
2
4
2
3
1
2
3
4
5
6
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9
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outstanding performance for deoxygenative CO reduction
with excellent CO-evolving rate (3075 μmol h g ) and high
stability.
junctions between the components. FESEM and TEM charac-
terizations for ZnIn S -In O samples collected at different
2
-1
-
1
2
4
2
3
reaction times demonstrate the possible epitaxial growth of
ZnIn S nanosheets on In O substrate (Figure S10). The het-
2
4
2
3
erojunction between few-layered ZnIn2S4 and In O3 is re-
2
vealed by HRTEM image (Figure 3e), in which the lattice
fringes assigned to crystal planes of cubic In O and hexago-
2
3
nal ZnIn S are discerned. Besides, the two sets of diffraction
2
4
fringes in SAED pattern further indicate the construction of
nanosized heterojunctions between ZnIn S and In O (Fig-
2
4
2
3
0
1
2
3
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ure 3f). EDX analysis determines the composition of ZnIn S -
2
4
In O with a ZnIn S molar ratio of ca. 40.3 % (Figure S11).
2
3
2 4
Elemental mappings of a single ZnIn S -In O microtube
2
4
2
3
reveal the even distribution of O, In, S and Zn (Figure 3g).
Such results reflect the formation of homogeneous length-
wise interfacial junctions between In O shell and ZnIn S
2 3 2 4
Figure 2. (a-c) FESEM images, (d,e) TEM images and (f) HRTEM
image of hexagonal In
2
O
3
microtubes.
layers. X-ray photoelectron spectroscopy (XPS) measure-
ments identify the valance states of elements in ZnIn S -
2
4
Hexagonal In-MIL-68 prisms are synthesized via a sol-
26
In
ulating composition of the final heterostructures to optimize
catalytic performance by controlling the amounts of ZnIn
precursors. The other two samples, ZnIn S -In O -0.5 (half
2 3
O (Figure S12). The developed recipe is effective for mod-
vothermal method. Powder X-ray diffraction (XRD) analy-
sis indicates the formation of In-based MIL-68 phase (Figure
26
S
2 4
S1). These In-MOFs present uniform hexagonal prism-
shaped morphology (Figure S2). Thermogravimetric analysis
2
4
2
3
precursor amounts, Figure S13-15) and ZnIn S -In O -2.0
2
4
2
3
(
TGA) reveals In-MIL-68 can be fully converted to In O after
2 3
(
double precursor amounts, Figure S16-18), with different
thermal annealing in air at 500 ˚C (Figure S3). After the
treatment, cubic In O is obtained without detectable impu-
compositions are synthesized. For comparison, the
nanosheet-assembled ZnIn S particles (Figure S19-S21) and
2
3
2
4
rities (Figure S4). Field-emission scanning electron micros-
copy (FESEM) images show the In O product preserves the
free-standing ZnIn S nanosheets (ZnIn S NSs, Figure S22)
2
4
2 4
2
3
27,28
are also prepared under similar conditions.
1D morphology as that of In-MIL-68 prisms (Figure 2a,b).
The magnified FESEM image demonstrates the well-defined
hexagonal open ends of the In O microtube (Figure 2c). The
2
3
hollow feature of the In O material is revealed by transmis-
2
3
sion electron microscopy (TEM) images (Figure 2d,e). Fur-
ther TEM analysis shows the shell of In O microtubes is
2
3
assembled from small nanocrystals interconnected firmly
with each other (Figure S5a,b), constructing the stable hol-
low structures with porous characteristics (Figure S6). In the
high-resolution TEM (HRTEM) image (Figure 2f), the lattice
fringes with interlayer distance of 0.29 nm are assigned to
(
222) crystal plane of cubic In O . The polycrystalline nature
2 3
of the In O material is indicated by selected area electron
2
3
diffraction (SAED) pattern (Figure S5c). The chemical com-
position and distribution of the In O microtubes are exam-
2
3
ined by energy-dispersive X-ray (EDX) spectroscopy (Figure
S7) and elemental mappings (Figure S5d).
Figure 3. (a-c) FESEM images, (d) TEM image, (e) HRTEM image
and (f) SAED patterns of ZnIn -In hierarchical microtubes, and
g) EDX mappings of an individual ZnIn -In microtube.
S
2 4
2 3
O
Then, two layers of ultrathin ZnIn S nanosheets are grown
2
4
(
S
2 4
2 3
O
on both inner and outer surfaces of the In O microtubes
2
3
27
through a low-temperature hydrothermal approach. After
the reaction, ZnIn S nanosheets are uniformly covered on
UV-Vis diffuse reflectance spectra (DRS) show In
hibits poor photoabsorption in the visible-light region. But
after growth of ZnIn nanosheets, the ZnIn -In com-
2 3
O ex-
2
4
the surfaces of In O microtubes with well-maintained over-
2
S
4
2
S
4
O
2 3
2
3
all 1D morphology and open ends (Figure 3a,b). The high-
magnification FESEM images show the ZnIn2S4 layers are
composed of randomly assembled nanosheets, and are grown
on both surfaces of In O microtubes (Figure 3c and Figure
posite displays strong visible-light absorption to ca. 500 nm
(Figure S23). From the Tauc plots (Figure S24), the bandgap
energies of In
be 2.73, 2.39 and 2.29 eV, respectively.
O , ZnIn S -In O and ZnIn S
are calculated to
Furthermore, the
2
3 2 4 2 3 2 4
3
,27
2
3
S8a-c), forming the sandwich-like ZnIn S -In O hierarchical
conduction band (CB) positions of the materials are deter-
mined by Mott-Schottky plots (Figure S25a-c). The derived
flat-band potentials of In O , ZnIn S -In O and ZnIn S are
2 3 2 4 2 3 2 4
ca. -0.78, -0.86 and -0.98 V (vs. normal hydrogen electrode,
NHE, pH = 7.0), which confirms their suitable redox poten-
2
4
2
3
tubular heterostructures. XRD analysis reveals ZnIn S -In O
3
2
4
2
contains the mixed phases of cubic In O3 and hexagonal
2
ZnIn2S4 (Figure S9). The hierarchical hollow structure of
ZnIn S -In O is revealed by TEM images (Figure 3d and Fig-
2
4
2
3
ure S8d). No visible interlayer gap is found between ZnIn2S4
tials to drive CO reduction reaction (Figure S25d). In addi-
2
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tion, N sorption tests reveal the porous features of ZnIn S -
(Figure 4d), confirming CO is the carbon source of CO for-
2
2
4
2
1
2
3
4
5
6
7
8
9
In O3 with a high Brunauer-Emmett-Teller (BET) surface
mation. To evaluate stability, ZnIn S -In O is repeatedly
2
2
4
2
3
2
-1
area of 128 m g (Figure S26), affording the material a maxi-
employed to run the CO reduction reaction for 5 cycles. No
2
3
-1
mum CO uptake of ca. 20 cm g at 0 °C (Figure S27), which
obvious deactivation is observed during the tests (Figure 4e),
highlighting the high stability of ZnIn S -In O . Besides, XRD
2
is about twice that of ZnIn S particles (Figure S28). All these
2
4
2
4
2
3
findings indicate the opportunity of ZnIn S -In O for visible-
and FESEM tests of used ZnIn S -In O after photocatalysis
2 4 2 3
also support the stability of the catalyst (Figure S29). Wave-
2
4
2
3
light CO photoreduction.
2
length-dependent CO reduction reactions are conducted on
2
ZnIn S -In O . The trend of CO/H evolution matches well
2
4
2
3
2
with photoabsorption of the heterojunction photocatalyst
(
Figure 4f), indicating the CO reduction reaction is induced
2
3
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by photoexcitation of ZnIn S -In O .
2
4
2
3
Figure 4. (a) CO
Time-yield plots of products. (c) CO
various conditions. (d) Results of GC-MS analysis of CO and CO.
e) Formation of CO/H in stability tests. (f) CO/H production un-
der light irradiation of different wavelengths.
2
photoreduction activities of different samples. (b)
2
reduction performance under
12
13
(
2
2
Photocatalytic CO reduction reactions are conducted in
2
2+
H O/acetonitrile mixture with Co(bpy)3
(bpy
= 2’2-
2
bipyridine) and triethanolamine (TEOA) as the cocatalyst
29
and electron donor, respectively. Bare In O is inactive to
2
3
Figure 5. (a) Time-resolved transient PL decay, (b) steady-state PL
spectra, (c) EIS spectra, and (d) transient photocurrent spectra of
ZnIn S -In O and ZnIn S .
2 4 2 3 2 4
reduce CO under visible-light irradiation (Figure 4a); while
2
the ZnIn S particles and ZnIn S NSs enable the deoxygena-
2
4
2 4
tive CO reduction with CO-evolving rates of 875 and 1275
2
-1 -1
μmol h g , respectively. However, the three heterojunction
Photoelectrochemical characterizations are performed to
elucidate the high CO reduction performance of ZnIn
In heterojunction photocatalyst. First, the specific charge
carrier dynamics of ZnIn -In and ZnIn particles are
probed by time-resolved photoluminescence (PL) spectros-
copy (Figure 5a). The average emission lifetime of ZnIn
In (2.54 ns) is shorter than that of ZnIn (3.50 ns).
photocatalysts manifest enhanced CO -to-CO conversion
2
S -
2 4
2
activities. Strikingly, the ZnIn S -In O sample displays the
O
2 3
2
4
2
3
-
1
-1
highest CO evolution rate of 3075 μmol h g . This CO re-
2
S
4
2
O
3
S
2 4
2
duction rate is comparable to that of many other CO con-
2
version systems (Table S1). Moreover, ZnIn S -In O also
S -
2 4
2
4
2
3
exhibits durable CO reduction activity during photoreaction
2
O
3
S
2 4
2
for 8 h (Figure 4b), affording an accumulated product yield
Meanwhile, steady-state PL spectra show the significant PL
quenching of the heterostructures (Figure 5b). The obvious
PL quenching and lifetime decrease indicate the formation of
4
-1
of ca. 4.56×10 μmol g . These results underline the remarka-
ble CO2 reduction performance of ZnIn S -In O , which
2
4
2
3
should be related to the unique structures and compositions
of the three-layered hierarchical tubular heterostructures for
boosting the separation and transfer of photoexcited charges.
an electron transfer pathway from ZnIn
2
S
4
to In
radiative quenching manner. Therefore, efficient interfacial
charge migration and separation are achieved in ZnIn
In . On the other hand, electrochemical impedance spec-
tra (EIS) reveal ZnIn -In manifests a smaller semicircu-
2 3
O in a non-
9
S -
2 4
Control experiments demonstrate the CO reduction per-
O
2 3
2
formance of a physical mixture of In O and ZnIn S is much
2
S
4
O
2 3
2
3
2 4
inferior to that of ZnIn S -In O (Figure 4c, column 1), indi-
lar in Nyquist plots (Figure 5c), suggesting a lower charge-
transfer resistance in the heterostructures that permits fast
transport and separation of photoinduced charges. Moreover,
2
4
2
3
cating the importance of intimate interfacial contact and
strong interactions in the heterostructures for the reaction.
Besides, if the CO reduction reaction is performed without
transient photocurrent spectra indicate ZnIn
2
S
4
-In
2
O
3
exhib-
2
photocatalyst or light irradiation, no CO/H can be detected
its a greatly enhanced photocurrent than ZnIn
2
S
4
particles
2
(
Figure 4c, column 2 and column 3), suggesting photocatalyt-
(Figure 5d), revealing the promoted generation and transfer
of photogenerated electron-hole pairs in the heterostructures.
All these observations demonstrate the ZnIn S -In O hierar-
2 4 2 3
chical hollow photocatalyst can enhance the generation, sep-
aration and transport of photoinduced charges, thus render-
ic nature of the reaction. Meanwhile, no obvious CO -to-CO
2
2+
conversion reaction is observed without TEOA or Co(bpy)3
30
(
Figure 4c, column 4 and column 5). In addition, when us-
ing Ar as the reaction gas, there is not any CO evolution de-
tected (Figure 4c, column 6). This result indicates the CO
ing high-efficiency of CO
In summary, sandwich-like ZnIn
tubular heterostructures are designed and assembled by
growing ZnIn nanosheets on both inner and outer surfaces
photoreduction.
2
product originates from splitting of CO molecules, which is
2
S
4
-In
O hierarchical
2 3
2
1
3
13
further verified by CO isotope experiment. Only CO is
detected when reaction is operated in a CO atmosphere
2
13
S
2 4
2
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of In O microtubes as double-heterojunction photocatalysts
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junction shells and ultrathin 2D nanosheet subunits remark-
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The experimental details, more FESEM/TEM images, XRD,
(15) Kuriki, R.; Matsunaga, H.; Nakashima, T.; Wada, K.;
Yamakata, A.; Ishitani, O.; Maeda, K. J. Am. Chem. Soc. 2016,
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Akatsuka, M.; Lu, D.; Yagi, S.; Yoshida, T.; Ishitani, O.;
Maeda, K. Angew. Chem. Int. Ed. 2017, 56, 4867.
EDX, DRS, Tauc plots, Mott-Schottky plots, N adsorption
2
isotherms, CO sorption isotherms. This material is available
2
free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
(
17) Guan, B. Y.; Yu, X. Y.; Wu, H. B.; Lou, X. W. Adv. Mater.
017, 29, 1703614.
(18) Wang, S.; Guan, B. Y.; Yu, L.; Lou, X. W. Adv. Mater. 2017,
9, 1702724.
Corresponding Author
2
Email: xwlou@ntu.edu.sg
2
(19) Guan, B. Y.; Yu, L.; Li, J.; Lou, X. W. Sci. Adv. 2016, 2,
Notes
e1501554.
The authors declare no competing financial interests.
(
(
20) Yu, L.; Wu, H. B.; Lou, X. W. Acc. Chem. Res. 2017, 50, 293.
21) Zheng, D.; Cao, X. N.; Wang, X. Angew. Chem. Int. Ed. 2016,
ACKNOWLEDGEMENTS
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22) Qiu, B.; Zhu, Q.; Du, M.; Fan, L.; Xing, M.; Zhang, J. Angew.
Chem. Int. Ed. 2017, 56, 2684.
(23) Sun, J.; Zhang, J.; Zhang, M.; Antonietti, M.; Fu, X.; Wang, X.
X. W. L. acknowledges the funding support from the Nation-
al Research Foundation (NRF) of Singapore via the NRF in-
vestigatorship (NRF-NRFI2016-04).
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24) Zhang, P.; Guan, B. Y.; Yu, L.; Lou, X. W. Chem 2018, 4, 162.
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